Exemplary embodiments of the invention relate to a catalytic converter component of a motor vehicle emission control system, and the use of such a catalytic converter component in an emission control system of a motor vehicle for heating exhaust gas.
PCT publication WO 01/74476 A1 discloses a catalytic converter component designed as a nitrogen oxides trap, which may have three coating zones, situated one downstream from the other, in the exhaust gas flow direction, having a decreasing content of metals of the platinum group. A high nitrogen oxides conversion capability over a wide temperature range is thus achieved.
Exemplary embodiments of the present invention are directed to providing a cost-effective catalytic converter component for a motor vehicle emission control system having a low overall precious metal content but still having high oxidation catalyst activity, in particular at comparatively low temperatures. Further, exemplary embodiments of the present invention are directed to providing an effective option for heating exhaust gas of a motor vehicle, in particular starting from low temperatures.
The catalytic converter component according to the invention includes a support body in a honeycomb design, having channels extending in a longitudinal direction and through which gas may flow freely from an inlet-side end of the catalytic converter component to an outlet-side end of the catalytic converter component. A catalytically active coating having a precious metal content determined by at least one element of the platinum group is applied throughout on the channel walls. A first coating zone extends in the longitudinal direction essentially from the inlet-side end to a first coating boundary situated in the area between the inlet-side end and the outlet-side end. In addition, a second coating zone extends in the longitudinal direction essentially from the first coating boundary to a second coating boundary situated in the area between the inlet-side end and the outlet-side end and downstream from the first coating boundary. In addition, a third coating zone extends in the longitudinal direction essentially from the second coating boundary to the outlet-side end. The coating of the third coating zone has a lower content of precious metals of the platinum group than the coating of the second coating zone, which in turn has a lower content of precious metals of the platinum group than the coating of the first coating zone. The coating of the catalytic converter component is characteristically formed as a coating having oxidation catalyst activity throughout and is free of rhodium. The first coating preferably has at least twice the precious metal content as the coating of the second coating zone, which in turn has at least twice the precious metal content as the third coating provided for the third coating zone. The precious metal content in one of the first and second coating zones is particularly preferably selected to be greater than 2.5 times that in the coating zone that immediately follows. In this regard, a factor of 3.5 is likewise considered to be advantageous.
Due to the zoning according to the invention having zones with a decreasing precious metal content, a comparatively low overall precious metal content may be achieved, whereby on account of the precious metal content that is present at the highest level on the inlet side, a very good light-off characteristic, i.e., a low light-off temperature, is achieved, in particular with regard to oxidation of hydrocarbons. Compared to designs in which a low light-off temperature is achieved by a high precious metal content throughout, the zoning according to the invention has the additional advantage of a reduced tendency toward oxidation of nitric oxide and sulfur dioxide. As a result of the design according to the invention as a catalytic converter component that is free of rhodium, the catalytic converter component can be manufactured in a particularly cost-effective manner. The coating is thus also designed as a coating having oxidation catalyst activity, of which the known three-way catalyst activity, achieved primarily by rhodium, preferably has very little, or at least to a greatly reduced extent. Also preferably absent are coating constituents having a storage effect with respect to nitrogen oxides and/or oxygen, or which are present only to a limited extent as a minor constituent. That is, at best, small fractions of metal compounds, in particular oxides, hydroxides, or carbonates of the alkali and alkaline earth groups, are present. The precious metals of the platinum group that are present, preferably represented primarily or solely by platinum and/or palladium, are finely dispersed or contained in a so-called wash coat that is preferably formed predominantly from the constituents aluminum oxide and/or zirconium oxide. A wash coat free of cerium oxide or which at best contains cerium oxide only to a limited extent is preferably provided. Apart from the outer surface of the honeycomb, the coating is present on all channel walls. The channel walls are preferably impermeable to gas, or at best have low gas permeability. In any case, a filtering effect is preferably present to a negligible extent, if at all.
A “coating extending essentially from the inlet-side end to the first coating boundary” is understood to mean that the first coating zone, apart from manufacturing-related tolerances that may be present, begins directly at the inlet-side end and ends at the first coating boundary. Similarly, the second coating zone, apart from small manufacturing-related tolerances, extends between the first and the second coating boundaries. The third coating zone, apart from small manufacturing-related tolerances, begins at the second coating boundary and extends directly to the outlet end. Thus, no, or at least no appreciable, areas without coating are present, and there is no, or at best negligible, overlap of coatings of the individual coating zones. Therefore, this may be considered to be an essentially overlap-free coating throughout of the catalytic converter component.
The support body is preferably designed as an extruded, one-piece ceramic support. However, a metal support body made of corrugated metal foils may also be provided. In particular in the case of a ceramic support body, the channels have a cross-section that is rectangular and that is at least approximately constant, viewed over the longitudinal direction. However, hexagonal or octagonal cross-sectional shapes may also be provided. The cell density, which is customarily expressed in cells per square inch (cpsi), is preferably in the range of 100 to 600. Cell densities between 200 cpsi and 400 cpsi are particularly preferred.
In one embodiment of the invention, the longitudinal extents of the first and the second coating zones are in each case 10% to 40% of the overall longitudinal extent of the catalytic converter component. The longitudinal extents of the first and the second coating zones are preferably in each case approximately 25% of the overall longitudinal extent of the catalytic converter component. The overall precious metal content may thus be kept low, since the third coating zone, having the lowest precious metal content, preferably constitutes at least 50% of the overall length.
In another embodiment of the invention, the precious metal content of the coating is determined by the elements platinum and palladium, the precious metal content being 12 g/ft3 to 100 g/ft3, based on the overall volume of the catalytic converter component. The average precious metal content is preferably 15 g/ft3 to 50 g/ft3. An average precious metal content is particularly preferably in the range of 25 g/ft3 to 40 g/ft3, as the result of which the catalytic converter component is particularly economical.
In another embodiment of the invention, the precious metals platinum and palladium are provided in the coating in a mass ratio of 1:5 to 5:1. A mass ratio of 1:2 to 2:1 is preferred. A mass ratio in the range of 1:1.5 to 1.5:1 is particularly preferred. This results in a high oxidation catalyst activity, in particular with regard to hydrocarbons.
In another embodiment of the invention, the precious metal content in the first coating zone is 60 g/ft3 to 150 g/ft3, in the second coating zone is 20 g/ft3 to 60 g/ft3, and in the third coating zone is 5 g/ft3 to 15 g/ft3. An embodiment having a precious metal content of approximately 80 g/ft3 in the first coating zone, approximately 30 g/ft3 in the second coating zone, and/or approximately 10 g/ft3 in the third coating zone is particularly preferred. Due to the comparatively high precious metal content that is present solely on the input side, a high level of cost savings may be achieved. The precious metal content is preferably uniformly distributed in a particular coating zone with respect to its longitudinal extent; i.e., there is preferably no, or at best only a small, gradient present in a particular coating zone.
The use according to the invention of the catalytic converter component in an emission control system of a motor vehicle provides heating of exhaust gas by oxidizing hydrocarbons upstream from upstream from an arrangement in series in the emission control system of a particle filter and an SCR catalytic converter downstream from the particle filter. As corroborated by testing, reliable heating, in particular starting from low exhaust gas temperatures, over a very long usage period is made possible. This makes the catalytic converter component particularly suitable for use in corresponding emission control systems of utility vehicles, since utility vehicles are typically used until they have high mileage. It is thus necessary to be able to reliably carry out particle filter regenerations by thermal soot burnoff over the longest possible usage periods, which typically involve several hundred thousand kilometers. It is typically necessary to carry out forced exhaust gas heating to a temperature that is required for soot burnoff, for which purpose the catalytic converter component is used.
In addition, it is desirable to achieve stable conditions, over the longest possible usage period, with regard to nitrogen dioxide formation at the catalytic converter component due to oxidation of nitric oxide contained in the exhaust gas, which influences the activity of the SCR catalytic converter. This is likewise made possible by the catalytic converter component according to the invention. Due to the excellent long-term stability of the catalytic converter component, an initially high oxidation power in this regard, which is often provided for oxidation catalytic converters via high platinum contents, may be intentionally dispensed with in order to compensate for an aging-related decrease in the nitrogen oxides oxidation activity.
In one particularly preferred embodiment of the invention, the catalytic converter component is used for oxidizing hydrocarbons that are at least predominantly present as diesel fuel. In this case, the diesel fuel is added to the exhaust gas upstream from the catalytic converter component by vaporization and/or spraying, and is oxidized by the catalytic converter component in an exothermic reaction, thus heating the exhaust gas.
Another embodiment of the invention provides for use of the catalytic converter component in an emission control system which downstream from the particle filter has a two-part SCR catalytic converter in the emission control system, the SCR catalytic converter, viewed in the exhaust gas flow direction, having a first portion with an iron-containing zeolite, and a second portion, downstream therefrom, with a copper-containing zeolite. As corroborated by testing, a high nitrogen oxides conversion rate with simultaneous low slip of nitrogen dioxide (NO2) is made possible due to a limited NO2 formation capacity of the catalytic converter component in conjunction with the two-part SCR catalytic converter. The two catalytic converter portions are preferably formed by separate honeycombs. However, the zeolite coatings in question may also be provided contiguously on the same honeycomb.
Advantageous embodiments of the invention are illustrated in the drawings and described below. The features mentioned above and to be explained below may be used not only in the particular stated feature combination, but also in other combinations or alone without departing from the scope of the present invention.